CE1313 – DESIGN OF STRUCTURES –II

Introduction / Unit I

by
YOGANANTHAM.C
M.Tech - Structural Engineering
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Syllabus
UNIT I - LIMIT STATE DESIGN FOR CONCRETE
STRUCTURE – Introduction
Limit state - characteristic load and characteristic
strength of materials - partial safety factor – stress-
strain relationship of concrete - safety and
serviceability requirements.

UNIT II - LIMIT STATE DESIGN OF BEAMS

Design of rectangular sections for bending -
singly reinforced, doubly reinforced and flanged
sections
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 UNIT III - LIMIT STATE DESIGN OF SLABS
Design of one-way and two-way slabs using IS
Code co-efficient for various edge conditions.

UNIT IV - LIMIT STATE DESIGN OF RCC COLUMNS 10

Behaviour of Columns - Code provisions - Design
of axially loaded short columns of rectangular and
circular sections - ties and spiral reinforcements.
Concept of Long columns (No Design calculations).

UNITV - WORKING STRESS DESIGN OF FOUNDATION

Types of foundations - Isolated pad footings for
simple design problems –Structural Concept of
3 combined footings (No Design calculations)
 TEXT BOOKS
1. P.C.Varghese, “Limit state Design of Reinforced
Concrete”, Prentice Hall of India , 2004.

2. Limit State Design of Reinforced Concrete, B.C

Pumia, A.K Jain, 2007

3. Reinforced Concrete Design, N.Krishnaraju &

R.N. Pranesh, New Age International Publications, 2006.
Code Books
 IS 456: 2000 - Plain and Reinforced Concrete - Code of
Practice
4  SP 16 - Design Aids For Reinforced Concrete to IS : 456
 Introduction
Types of Concrete
Plain Cement Concrete (PCC)
 Solid mass made of cement, sand, gravel, cement and water
 Durability, ease in casting, easy availability and economical
 Good in compression over brick and stone masonry
 Density ranges from 1200 to 2500 kg/m3
 Compressive strength: 10 to 100 N/mm2
 Very weak in tension, so limited use as structural material
 Used in hollow/solid blocks, small pedestals, mass concrete
applications like dams, etc.
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 Reinforced Cement Concrete (RCC)
 Concrete with steel bars embedded in it
 Composite material with resistance to tensile stresses
 Steel bars provided are in tension zone of flexural members
 Bond b/t steel & concrete ensures strain compatibility
 Steel imparts ductility to the brittle concrete
 Tensile stresses arise as direct tension, flexural tension, shear
force, temperature and shrinkage effects
 Steel will also resist compressive stresses along with
concrete

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 Pre-stressed Concrete
 High-strength concrete with high tensile steel wires
embedded and tensioned, prior to loading
 Concrete pre-compressed to a degree that, after loading
there is no resultant tension developed in the beam
 Used in bridges, tanks, railway sleepers, etc.

Fibre-Reinforced Concrete: Steel/glass fibres are

incorporated in concrete at the time of mixing

Ferrocement: Thin sections are formed by embedding

multiple layers of steel wire mesh in cement mortar
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 Objectives Of Structural Design
 Stability to prevent overturning, sliding or buckling under
loads
 Strength to resist safely the stresses induced by the loads
 Serviceability to ensure satisfactory performance under
service load conditions —providing adequate stiffness and
reinforcements to contain deflections, crack-widths and
vibrations within acceptable limits, and also providing
impermeability and durability (including corrosion-
resistance), etc.

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 Load Transfer Mechanism

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 Load Transfer Mechanism

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 Flooring Systems - Wall Supported Slab System

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 Beam-supported Slab Systems

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 Ribbed Slab Systems

One Way Ribbed Slab Two Way Ribbed Slab

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 Flat Plate and Flat Slab Systems

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 Vertical Load Resisting Systems
Columns
 These are skeletal structural elements, whose cross-sectional
shapes may be rectangular, square, circular, L-shaped, etc.
 Forces: Axial, bending moments and lateral shear forces

Walls
 Bearing walls – resist gravity loads
 Shear walls – resist lateral loads

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 Transfer Girders & Suspenders

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 Lateral Load Resisting Systems

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 Lateral Load Resisting Systems

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 Structural Analysis And Design
 Analysis: to determine the stress resultants and
displacements in the various members of a structure under
any loading
 Design: to provide adequate member sizes, reinforcement
and connection details, to withstand safely the load
 To perform analysis, the proportions of various structural
elements should be known in advance; for this, a preliminary
design is generally required
 In practice, analysis and design are interactive processes

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 Design Components in this Course
Working Stress Method & Limit State Design
Beam: Singly & Doubly Reinforced Rectangular Beam, T
Beam, L Beam
Column: Axially Loaded, Uniaxial Bending And Biaxial
Bending Short and Long Columns
Slabs: One Way and Two Way Slabs

Limit State Design

Footings: Square And Rectangular Footing for Axially and
Eccentrically Loaded Columns, Combined Footing

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 Basic Material Properties
 Cement
 Fine Aggregate
 Coarse Aggregate
 Water
 Admixtures
 Reinforcing Steel

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 Grade of Concrete
 Compressive Strength can be easily obtained and correlated
with other properties of concrete
 The quality or grade of concrete is designated in terms of its
characteristic compressive strength (of 150 mm cubes at 28-
days), expressed in MPa
 The number is usually preceded by the letter ‘M’, which
refers to ‘mix’.
 Ex. M25 - characteristic strength of 25 Mpa
 Characteristic strength: strength of material below which
not more than 5% of test results are expected to fall
 Table 2, IS 456: 2000 gives standard grades used
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 Table 2, Grades of Concrete

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 Quality Control in Concrete

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 Idealised normal distribution of concrete strength

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 Stress – Strain in Concrete

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 Elastic Modulus of Concrete
 As per Cl. 6.2.3.1, IS 456: 2000, E = 5000√fck

Tensile Strength of Concrete

 As per Cl. 6.2.2, IS 456: 2000, fcr = 0.7√fck

Poisson’s Ration
 the ratio of the lateral strain to the longitudinal strain, under
uniform axial stress
 Ranges from 0.1 to 0.3, normally 0.2 is taken

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 Reinforcing Steel
As per Cl. 5.6 of IS 456, steel used are
 Mild steel and medium tensile steel bars conforming to IS
432 (Part 1)
 High yield strength deformed (HYSD) steel bars conforming
to IS 1786
 Hard-drawn steel wire fabric conforming to IS 1566
 Structural steel conforming to Grade A of IS 2062

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 Diameter of bars
 Nominal diameters, mm: 6,8,10,12,16,20,25,28,32,36,40
 Table 1 of IS 1786:2008 gives the area and mass of bars

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 Stress-Strain Curves

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 Stress-Strain Curves

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 Loads and Combinations
Primary Loads
 Dead Load
 Imposed Load (Live Load)
 Earthquake Load
 Wind Load
 Earth pressure
 Temperature Load
 Settlement of Supports
 Shrinkage effects

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 Types of Loads

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 Dead Loads
 Fixed in magnitude and position
 Dead load of a structure: weight of structure together with
its associated ‘non-structural’ components
 After the design process, the initially assumed dead load of
the structure has to be compared with the actual dead load
 If difference is significant, revise the assumed dead load
Imposed Loads
Imposed loads (also referred to as live loads) are gravity loads
other than dead loads and include items such as occupancy by
people, movable equipment and furniture within the buildings,
stored materials such as books or machinery
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 Dead Loads (IS 875 (Part 1): 1987

No. Material Unit Weight

1. Brick masonry in CM 1:4 20 kN/m3
2. Plain concrete 24 kN/m3
3. Reinforced cement concrete 25 kN/m3
4. Stone masonry 20.4–26.5 kN/m3
5. Cement mortar 20.4 kN/m3
6. Steel 78.5 kN/m3
7. 20 mm cement plaster 450 N/m2
8. 5 mm glass 125 N/m2
9. Floor finishes 600–1200 N/m2
10. Water 10 kN/m3
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 Imposed Loads (IS 875 (Part 2): 1987
No. Type of Floor Usage Imposed Load
1. Residential 2.0 kN/m2
2. Office
(a) with separate storage 2.5 kN/m2
(b) without separate storage 4.0 kN/m2
3. Shops, classrooms, restaurants, theatres, etc.
(a) with fixed seating 4.0 kN/m2
(b) without fixed seating 5.0 kN/m2
4. Factories and warehouses 5.0-10.0 kN/m2
5. Book stores and stack rooms in libraries 10.0 kN/m2
6. Garages with light vehicles 4.0 kN/m2
7. Stairs, landings, and balconies
(a) not liable to overcrowding 4.0 kN/m2
38 (b) liable to overcrowding 5.0 kN/m2
 Wind Loads
 Code IS 875:1987 (Part 3) provides the basic wind speeds,
averaged over a short interval of 3 seconds and having a 50-
year return period at 10 m height above ground level in
different parts of the country
 Wind Load Depends on: Velocity and density of air, Height
above ground level, Shape and aspect ratio of the building,
Topography of the surrounding ground surface, Angle of
wind attack, Solidity ratio or openings in the structure,
Susceptibility of the structural system under consideration
to steady and time-dependent (dynamic) effects induced by
the wind load

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 Wind Zones

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 Earthquake Loads
 Calculated using IS 1893 (Part 1): 2002
 Depends on seismic weight of the building and seismic
parameters like zone factor, importance factor, response
reduction factor, soil properties
 Dynamic analysis has to adopted as per code

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 Seismic Zones

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 Load Combinations
Limit State of Collapse
 1.5 (DL+IL)
 1.2(DL+IL±EL)
 1.5(DL±EL)
 1.5(DL±EL)

Limit State of Serviceability

 1.0 (DL+IL)
 1.0DL + 0.8(IL±EL)
 1.0(DL±EL)

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 Design Concepts
Design Considerations
Safety
 Partial or total collapse is acceptably low under normal
service loads and abnormal but probable overloads
(earthquake or wind)
 Collapse occurs due to exceeding the load−bearing capacity,
overturning, sliding, buckling, fatigue fracture, etc.
 Structural integrity should maintained
 Minimise the likelihood of progressive collapse

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 Serviceability
 Satisfactory performance under service loads, without
discomfort to the user due to excessive deflection, cracking,
vibration, etc.
 Other considerations are durability, impermeability, acoustic
and thermal insulation, etc.
 A design that satisfies safety requirement need not
necessarily satisfy serviceability requirement
 Example: a thin reinforced concrete slab can be safe against
collapse but it is likely to result in excessive deflections,
crack-widths and permeability and the exposed steel
becomes vulnerable to corrosion
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 Design Philosophies
Working Stress Method (WSM)
 Traditional method used in RCC, steel & timber structures
 Structural material behaves in a linear elastic manner
 Adequate safety is ensured by suitably restricting stresses in
the material induced by working loads
 As permissible stresses are kept well below the material
strength, assumption of linear elastic behaviour is justified
 Factor of Safety: ratio of material strength to permissible
stress
 Strain Compatibility due to bond is assumed

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  Stresses in concrete and steel are assumed to be linearly
related to their respective strains
 stress in steel is linearly related to that in the adjoining
concrete by modular ratio (Es/Ec)
 Stresses under working loads kept within permissible
stresses are not realistic due to long-term effects of creep
and shrinkage, effects of stress concentrations, and other
secondary effects, resulting redistribution
 WSM fails to discriminate between different types of loads
that act simultaneously, but have different degrees of
uncertainty leading to un-conservative designs
 Structures designed usingWSM are performing satisfactorily
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 Limit State Method
 Comprehensive and rational solution to the design problem,
by considering safety at ultimate loads and serviceability at
working loads
 Uses a multiple safety factor format which attempts to
provide adequate safety at ultimate loads as well as adequate
serviceability at service loads, by considering all possible
‘limit states’

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 Limit states
 State of impending failure, beyond which a structure ceases
to perform its intended function satisfactorily, in terms of
either safety or serviceability; i.e., it either collapses or
becomes unserviceable
 Ultimate limit states (or ‘limit states of collapse’), which
deal with strength, overturning, sliding, buckling, fatigue
fracture, etc.
 Serviceability limit states, which deal with discomfort
to occupancy and/or malfunction, caused by excessive
deflection, crack-width, vibration, leakage, etc., and also
loss of durability, etc.

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